fix: coordinate sqlite index writers

Implement the full ADR-0006 plan: three-layer separation of transaction
correctness, retry policy, and cross-process writer coordination.

Layer 1 — scripts/tx.ts (transaction correctness):
- runWriteTransaction executes work exactly once; no internal retry.
- BEGIN IMMEDIATE takes the write lock up front (avoids SQLITE_BUSY_SNAPSHOT).
- Guarded rollback: checks inTransaction() via adapter before attempting
  ROLLBACK; never masks the primary exception.
- WriteTxDiagnostics attached to errors: phase, code, label,
  rollbackSucceeded, rollbackError, transactionActive.
- Binding adapters (betterSqliteTransactionAdapter, nodeSqliteTransactionAdapter)
  mapping better-sqlite3's `.inTransaction` and node:sqlite's `.isTransaction`.
- configureConnection centralizes WAL + synchronous + busy_timeout.

Layer 2 — scripts/write-coordinator.ts (retry policy):
- runRetryableWriteTransaction: bounded retry with total time budget.
- Only retries when the transaction confirmed ended (transactionActive=false)
  and the error is SQLITE_BUSY during work/commit phase.
- BEGIN-phase BUSY = abort entire build (isBeginBusyFailure); the caller
  returns `{ deferred: true, reason: 'writer_busy' }` instead of waiting.
- hasUnusableTransaction detects a still-active transaction after failure;
  aborts the build immediately, never retries.

Layer 3 — scripts/writer-lease.ts (cross-process coordination):
- acquireWriterLease: dedicated writer.lock.sqlite with busy_timeout=0 +
  BEGIN IMMEDIATE. Non-blocking attempt; bounded wait with retryDelayMs.
- writerLockPathFor derives lock path from the target DB path.
- Lease held for the entire build; released on completion or failure.
- Lock DB uses DELETE journal (not WAL); crash/close auto-releases.
- All consumers obey: skill acquires at build start (returns deferred if
  unavailable); app daemon (via worker) acquires for its build cycle.

Build semantics changes:
- affectedSessionIds updated only after successful commit.
- BuildIndexResult gains skipped/skippedFiles for observability.
- Skill finalize failure now fails the build (was silently warned).
- Checkpoint changed to PASSIVE (TRUNCATE reserved for maintenance/exit).
- Skill buildIndex returns { deferred, reason } on lease contention;
  indexer-service reschedules the build (deferredRetryMs) without publishing
  a heartbeat (so the build-deferred state is visible to cross-process
  arbitration).
- Service publishes heartbeat immediately on start() for correct arbitration.

Tests:
- tests/write-transaction.test.mjs: single-shot execution, diagnostics
  propagation, auto-rolled-back transaction detected, rollback failure
  captured as metadata, BEGIN IMMEDIATE semantics.
- tests/writer-lease.test.mjs: acquire/release, contention returns null,
  bounded wait with release during budget.
- tests/app-writer-lease.test.mjs: better-sqlite3 adapter integration.
- tests/app-rollback-guard.test.mjs: rewritten — transient BUSY recovered
  by coordinator, persistent BUSY skips file, begin-busy aborts build,
  live-transaction aborts build, phantom affectedSessionIds prevented.
- tests/daemon-arbitration.test.mjs: skill defers to fresh app heartbeat,
  builds when heartbeat is stale.
- tests/app-indexer-service.test.mjs: new cases for deferred-retry
  scheduling and immediate heartbeat on start.
- app/tests/electron-concurrency.mjs + child: dual-child IPC structure for
  real better-sqlite3 contention (holder acquires lock → build child starts
  → delayed release → result collected; persistent contention bounded).

ADR-0006 updated to reflect the implemented design.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/app-indexer-service.test.mjs

@@ -73,6 +73,34 @@ test('indexer service runs one pending build after an in-flight build finishes',
   assert.deepEqual(calls, ['first', 'pending']);
 });
 
+test('indexer service reschedules a writer-lease deferral without publishing a heartbeat', async () => {
+  const timers = manualTimers();
+  const calls = [];
+  let heartbeats = 0;
+  const service = createIndexerService({
+    buildIndex: async ({ reason, changedPaths }) => {
+      calls.push({ reason, changedPaths });
+      return calls.length === 1 ? { deferred: true, reason: 'writer_busy' } : { deferred: false };
+    },
+    watchProjects: () => null,
+    writeHeartbeat: () => { heartbeats += 1; },
+    timers,
+    stabilityMs: 0,
+  });
+
+  await service.runBuildNow('watch', ['project/session.jsonl']);
+  assert.equal(heartbeats, 0);
+  assert.equal(calls.length, 1);
+
+  timers.flush();
+  await service.idle();
+  assert.deepEqual(calls, [
+    { reason: 'watch', changedPaths: ['project/session.jsonl'] },
+    { reason: 'writer-lease', changedPaths: ['project/session.jsonl'] },
+  ]);
+  assert.equal(heartbeats, 1);
+});
+
 test('indexer service does not log a build cancelled by a service stop', async () => {
   const timers = manualTimers();
   const warnings = [];
@@ -160,6 +188,22 @@ test('indexer service retries watcher setup when the projects directory is missi
   assert.equal(attempts, 2);
 });
 
+test('indexer service publishes daemon ownership as soon as it starts', () => {
+  const timers = manualTimers();
+  let heartbeats = 0;
+  const service = createIndexerService({
+    buildIndex: async () => ({ deferred: false }),
+    watchProjects: () => null,
+    writeHeartbeat: () => { heartbeats += 1; },
+    timers,
+    stabilityMs: 0,
+  });
+
+  service.start({ buildOnStart: false });
+  assert.equal(heartbeats, 1);
+  service.stop();
+});
+
 test('indexer service watches Claude JSON files through chokidar', async () => {
   const projectsDir = mkdtempSync(join(tmpdir(), 'obelisk-chokidar-projects-'));
   const timers = manualTimers();